Lei Shu


2022

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Multi-Task Pre-Training for Plug-and-Play Task-Oriented Dialogue System
Yixuan Su | Lei Shu | Elman Mansimov | Arshit Gupta | Deng Cai | Yi-An Lai | Yi Zhang
Proceedings of the 60th Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers)

Pre-trained language models have been recently shown to benefit task-oriented dialogue (TOD) systems. Despite their success, existing methods often formulate this task as a cascaded generation problem which can lead to error accumulation across different sub-tasks and greater data annotation overhead. In this study, we present PPTOD, a unified plug-and-play model for task-oriented dialogue. In addition, we introduce a new dialogue multi-task pre-training strategy that allows the model to learn the primary TOD task completion skills from heterogeneous dialog corpora. We extensively test our model on three benchmark TOD tasks, including end-to-end dialogue modelling, dialogue state tracking, and intent classification. Experimental results show that PPTOD achieves new state of the art on all evaluated tasks in both high-resource and low-resource scenarios. Furthermore, comparisons against previous SOTA methods show that the responses generated by PPTOD are more factually correct and semantically coherent as judged by human annotators.

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TaCL: Improving BERT Pre-training with Token-aware Contrastive Learning
Yixuan Su | Fangyu Liu | Zaiqiao Meng | Tian Lan | Lei Shu | Ehsan Shareghi | Nigel Collier
Findings of the Association for Computational Linguistics: NAACL 2022

Masked language models (MLMs) such as BERT have revolutionized the field of Natural Language Understanding in the past few years. However, existing pre-trained MLMs often output an anisotropic distribution of token representations that occupies a narrow subset of the entire representation space. Such token representations are not ideal, especially for tasks that demand discriminative semantic meanings of distinct tokens. In this work, we propose TaCL (Token-aware Contrastive Learning), a novel continual pre-training approach that encourages BERT to learn an isotropic and discriminative distribution of token representations. TaCL is fully unsupervised and requires no additional data. We extensively test our approach on a wide range of English and Chinese benchmarks. The results show that TaCL brings consistent and notable improvements over the original BERT model. Furthermore, we conduct detailed analysis to reveal the merits and inner-workings of our approach.

2021

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CLASSIC: Continual and Contrastive Learning of Aspect Sentiment Classification Tasks
Zixuan Ke | Bing Liu | Hu Xu | Lei Shu
Proceedings of the 2021 Conference on Empirical Methods in Natural Language Processing

This paper studies continual learning (CL) of a sequence of aspect sentiment classification (ASC) tasks in a particular CL setting called domain incremental learning (DIL). Each task is from a different domain or product. The DIL setting is particularly suited to ASC because in testing the system needs not know the task/domain to which the test data belongs. To our knowledge, this setting has not been studied before for ASC. This paper proposes a novel model called CLASSIC. The key novelty is a contrastive continual learning method that enables both knowledge transfer across tasks and knowledge distillation from old tasks to the new task, which eliminates the need for task ids in testing. Experimental results show the high effectiveness of CLASSIC.

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古汉语词义标注语料库的构建及应用研究(The Construction and Application of Ancient Chinese Corpus with Word Sense Annotation)
Lei Shu (舒蕾) | Yiluan Guo (郭懿鸾) | Huiping Wang (王慧萍) | Xuetao Zhang (张学涛) | Renfen Hu (胡韧奋)
Proceedings of the 20th Chinese National Conference on Computational Linguistics

古汉语以单音节词为主,其一词多义现象十分突出,这为现代人理解古文含义带来了一定的挑战。为了更好地实现古汉语词义的分析和判别,本研究基于传统辞书和语料库反映的语言事实,设计了针对古汉语多义词的词义划分原则,并对常用古汉语单音节词进行词义级别的知识整理,据此对包含多义词的语料开展词义标注。现有的语料库包含3.87万条标注数据,规模超过117.6万字,丰富了古代汉语领域的语言资源。实验显示,基于该语料库和BERT语言模型,词义判别算法准确率达到80%左右。进一步地,本文以词义历时演变分析和义族归纳为案例,初步探索了语料库与词义消歧技术在语言本体研究和词典编撰等领域的应用。

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ODIST: Open World Classification via Distributionally Shifted Instances
Lei Shu | Yassine Benajiba | Saab Mansour | Yi Zhang
Findings of the Association for Computational Linguistics: EMNLP 2021

In this work, we address the open-world classification problem with a method called ODIST, open world classification via distributionally shifted instances. This novel and straightforward method can create out-of-domain instances from the in-domain training instances with the help of a pre-trained generative language model. Experimental results show that ODIST performs better than state-of-the-art decision boundary finding method.

2020

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Understanding Pre-trained BERT for Aspect-based Sentiment Analysis
Hu Xu | Lei Shu | Philip Yu | Bing Liu
Proceedings of the 28th International Conference on Computational Linguistics

This paper analyzes the pre-trained hidden representations learned from reviews on BERT for tasks in aspect-based sentiment analysis (ABSA). Our work is motivated by the recent progress in BERT-based language models for ABSA. However, it is not clear how the general proxy task of (masked) language model trained on unlabeled corpus without annotations of aspects or opinions can provide important features for downstream tasks in ABSA. By leveraging the annotated datasets in ABSA, we investigate both the attentions and the learned representations of BERT pre-trained on reviews. We found that BERT uses very few self-attention heads to encode context words (such as prepositions or pronouns that indicating an aspect) and opinion words for an aspect. Most features in the representation of an aspect are dedicated to the fine-grained semantics of the domain (or product category) and the aspect itself, instead of carrying summarized opinions from its context. We hope this investigation can help future research in improving self-supervised learning, unsupervised learning and fine-tuning for ABSA. The pre-trained model and code can be found at https://github.com/howardhsu/BERT-for-RRC-ABSA.

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DomBERT: Domain-oriented Language Model for Aspect-based Sentiment Analysis
Hu Xu | Bing Liu | Lei Shu | Philip Yu
Findings of the Association for Computational Linguistics: EMNLP 2020

This paper focuses on learning domain-oriented language models driven by end tasks, which aims to combine the worlds of both general-purpose language models (such as ELMo and BERT) and domain-specific language understanding. We propose DomBERT, an extension of BERT to learn from both in-domain corpus and relevant domain corpora. This helps in learning domain language models with low-resources. Experiments are conducted on an assortment of tasks in aspect-based sentiment analysis (ABSA), demonstrating promising results.

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Controllable Text Generation with Focused Variation
Lei Shu | Alexandros Papangelis | Yi-Chia Wang | Gokhan Tur | Hu Xu | Zhaleh Feizollahi | Bing Liu | Piero Molino
Findings of the Association for Computational Linguistics: EMNLP 2020

This work introduces Focused-Variation Network (FVN), a novel model to control language generation. The main problems in previous controlled language generation models range from the difficulty of generating text according to the given attributes, to the lack of diversity of the generated texts. FVN addresses these issues by learning disjoint discrete latent spaces for each attribute inside codebooks, which allows for both controllability and diversity, while at the same time generating fluent text. We evaluate FVN on two text generation datasets with annotated content and style, and show state-of-the-art performance as assessed by automatic and human evaluations.

2019

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Modeling Multi-Action Policy for Task-Oriented Dialogues
Lei Shu | Hu Xu | Bing Liu | Piero Molino
Proceedings of the 2019 Conference on Empirical Methods in Natural Language Processing and the 9th International Joint Conference on Natural Language Processing (EMNLP-IJCNLP)

Dialogue management (DM) plays a key role in the quality of the interaction with the user in a task-oriented dialogue system. In most existing approaches, the agent predicts only one DM policy action per turn. This significantly limits the expressive power of the conversational agent and introduces unwanted turns of interactions that may challenge users’ patience. Longer conversations also lead to more errors and the system needs to be more robust to handle them. In this paper, we compare the performance of several models on the task of predicting multiple acts for each turn. A novel policy model is proposed based on a recurrent cell called gated Continue-Act-Slots (gCAS) that overcomes the limitations of the existing models. Experimental results show that gCAS outperforms other approaches. The datasets and code are available at https://leishu02.github.io/.

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Flexibly-Structured Model for Task-Oriented Dialogues
Lei Shu | Piero Molino | Mahdi Namazifar | Hu Xu | Bing Liu | Huaixiu Zheng | Gokhan Tur
Proceedings of the 20th Annual SIGdial Meeting on Discourse and Dialogue

This paper proposes a novel end-to-end architecture for task-oriented dialogue systems. It is based on a simple and practical yet very effective sequence-to-sequence approach, where language understanding and state tracking tasks are modeled jointly with a structured copy-augmented sequential decoder and a multi-label decoder for each slot. The policy engine and language generation tasks are modeled jointly following that. The copy-augmented sequential decoder deals with new or unknown values in the conversation, while the multi-label decoder combined with the sequential decoder ensures the explicit assignment of values to slots. On the generation part, slot binary classifiers are used to improve performance. This architecture is scalable to real-world scenarios and is shown through an empirical evaluation to achieve state-of-the-art performance on both the Cambridge Restaurant dataset and the Stanford in-car assistant dataset.

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BERT Post-Training for Review Reading Comprehension and Aspect-based Sentiment Analysis
Hu Xu | Bing Liu | Lei Shu | Philip Yu
Proceedings of the 2019 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies, Volume 1 (Long and Short Papers)

Question-answering plays an important role in e-commerce as it allows potential customers to actively seek crucial information about products or services to help their purchase decision making. Inspired by the recent success of machine reading comprehension (MRC) on formal documents, this paper explores the potential of turning customer reviews into a large source of knowledge that can be exploited to answer user questions. We call this problem Review Reading Comprehension (RRC). To the best of our knowledge, no existing work has been done on RRC. In this work, we first build an RRC dataset called ReviewRC based on a popular benchmark for aspect-based sentiment analysis. Since ReviewRC has limited training examples for RRC (and also for aspect-based sentiment analysis), we then explore a novel post-training approach on the popular language model BERT to enhance the performance of fine-tuning of BERT for RRC. To show the generality of the approach, the proposed post-training is also applied to some other review-based tasks such as aspect extraction and aspect sentiment classification in aspect-based sentiment analysis. Experimental results demonstrate that the proposed post-training is highly effective.

2018

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Double Embeddings and CNN-based Sequence Labeling for Aspect Extraction
Hu Xu | Bing Liu | Lei Shu | Philip S. Yu
Proceedings of the 56th Annual Meeting of the Association for Computational Linguistics (Volume 2: Short Papers)

One key task of fine-grained sentiment analysis of product reviews is to extract product aspects or features that users have expressed opinions on. This paper focuses on supervised aspect extraction using deep learning. Unlike other highly sophisticated supervised deep learning models, this paper proposes a novel and yet simple CNN model employing two types of pre-trained embeddings for aspect extraction: general-purpose embeddings and domain-specific embeddings. Without using any additional supervision, this model achieves surprisingly good results, outperforming state-of-the-art sophisticated existing methods. To our knowledge, this paper is the first to report such double embeddings based CNN model for aspect extraction and achieve very good results.

2017

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Lifelong Learning CRF for Supervised Aspect Extraction
Lei Shu | Hu Xu | Bing Liu
Proceedings of the 55th Annual Meeting of the Association for Computational Linguistics (Volume 2: Short Papers)

This paper makes a focused contribution to supervised aspect extraction. It shows that if the system has performed aspect extraction from many past domains and retained their results as knowledge, Conditional Random Fields (CRF) can leverage this knowledge in a lifelong learning manner to extract in a new domain markedly better than the traditional CRF without using this prior knowledge. The key innovation is that even after CRF training, the model can still improve its extraction with experiences in its applications.

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DOC: Deep Open Classification of Text Documents
Lei Shu | Hu Xu | Bing Liu
Proceedings of the 2017 Conference on Empirical Methods in Natural Language Processing

Traditional supervised learning makes the closed-world assumption that the classes appeared in the test data must have appeared in training. This also applies to text learning or text classification. As learning is used increasingly in dynamic open environments where some new/test documents may not belong to any of the training classes, identifying these novel documents during classification presents an important problem. This problem is called open-world classification or open classification. This paper proposes a novel deep learning based approach. It outperforms existing state-of-the-art techniques dramatically.

2016

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Lifelong-RL: Lifelong Relaxation Labeling for Separating Entities and Aspects in Opinion Targets
Lei Shu | Bing Liu | Hu Xu | Annice Kim
Proceedings of the 2016 Conference on Empirical Methods in Natural Language Processing